1. Technical Field
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to an LCD device of a single cell gap transmitting-reflective type.
2. Description of the Related Art
LCD devices operate by using optical anisotropy and polarization properties of a liquid crystal material. Liquid crystal molecules have long and thin shapes and tend to align in the same direction according to an electric field. A suitable electric field controls the alignment direction of the liquid crystal molecules. The optical anisotropy of the liquid crystal material is changed and light propagating through the liquid crystal material is polarized. Accordingly, an image is displayed.
LCD devices include a thin film transistor array substrate with a thin film transistor and a pixel electrode; a color filter array substrate with a color filter layer; and a liquid crystal layer formed between the two substrates. Recently, an active matrix (AM) type LCD device has become popular due to high resolution and good picture quality. The AM type LCD device includes the thin film transistor and the pixel electrode arranged in a matrix configuration.
LCD devices do not emit the light. LCD devices use an additional light source, for example, a backlight unit. The amount of light viewed through LCD devices is about 7% of a total amount of light generated from the backlight unit. A high luminance LCD device may need a large amount of light, which may increase power consumption of the backlight unit. A heavy battery is required to power the backlight unit. The operating time of the backlight unit may be limited in a battery mode.
In bright surroundings, it may be difficult to recognize images displayed on LCD devices. Accordingly, a transmitting-reflective (referred to herein as “transflective”) type LCD device is under development. The transflective type LCD device also may be referred to as a transreflective type LCD device. The transflective type LCD device may use both ambient light and the light generated from the backlight. The transflective type LCD device includes unit pixel regions, and each of the unit pixels has a transmitting part and a reflective part.
FIG. 1 illustrates a Vertical Assignment (VA) mode transflective type LCD device of the related art. FIG. 1 is a cross section view illustrating a related art VA mode transflective type LCD device 5. As shown in FIG. 1, the LCD device 5 includes a unit cell divided into a reflective part and a transmitting part. A cell gap of the reflective part is different from a cell gap of transmitting part, which is referred to as a dual cell gap structure. The dual cell gap structure is designed to provide a different birefringence to the reflective part and the transmitting part.
In FIG. 1, first and second substrates 10 and 30 are opposite to each other, and a liquid crystal layer 50 is formed between the first and second substrates 10 and 30. A backlight unit emits the light below the first substrate 10. In addition, alignment layers are formed on facing surfaces of the first and second substrates 10 and 30. Liquid crystal molecules of the liquid crystal layer 50 are aligned at a predetermined direction.
The first substrate 10 includes gate and data lines crossing each other to define a pixel region; a thin film transistor formed adjacent to the crossing of the gate and data lines; a passivation layer formed on the thin film transistor; a reflective sheet 11 formed on the passivation layer of reflective part so as to reflect the ambient light (natural or artificial light); an insulation layer 12 formed on an entire surface of the passivation layer including the reflective sheet 11; and a pixel electrode 13 of a transparent material formed on the insulation layer 12 and connected with a drain electrode of the thin film transistor. The pixel electrode 13 is provided with slit patterns 13a to divide the unit pixel region into multi-domains.
The second substrate 30 includes a black matrix layer to prevent light leakage on other portions except the pixel regions; an R/G/B color filter layer 32 to represent colors in the respective pixel regions; a common electrode 34 formed on the R/G/B color filter layer 32; and an overcoat layer 36 formed on the common electrode 34 of the reflective part.
In the reflective part, the ambient light passes through the liquid crystal layer 50 from the second substrate 30, and is then reflected on the reflective sheet 11, and again passes through the liquid crystal layer 50. Accordingly, the light passes through the liquid crystal layer 50 twice. For the transmitting part, the light passes through the liquid crystal layer once. In this case, the cell gap (g1) of the reflective part is different from the cell gap (g2) of transmitting part. The overcoat layer 36 creates two different cell gaps (g1) and (g2). The different cell gaps (g1) and (g2) also results in different phases of the light which passes the reflective part and the transmitting part.
Optical properties of the transmitting and the reflective parts may become consistent by controlling the thickness of the overcoat layer 36 formed on the common electrode 34 of the reflective part. However, additional processes of depositing and patterning the overcoat layer 36 to stay only on the reflective part may be performed. The gap difference is also generated between the transmitting part and the reflective part due to the overcoat layer 36. The gap difference affects an alignment layer deposited on the entire surface of the substrate including the overcoat layer 36 and a rubbing process. Rubbing defects may occur. Accordingly, there is a need for a transflective type LCD device that substantially obviates drawbacks of the related art.